1. Field of the Invention
The present invention relates to polyethylene terephthalate bottles having improved gas barrier properties. Particularly, this invention relates to a gas barrier label for polyethylene terephthalate bottles.
2. Description of the Prior Art
The wide use of food and beverage containers fabricated from polyethylene terephthalate (PET) is due to the good balance of physical and mechanical properties of the polymer, including clarity, formability, gas and moisture barrier properties, and inertness to a wide variety of chemicals. Such desirable characteristics of PET have led to its wide acceptance as a carbonated-beverage bottle material. In addition, the carbonated-beverage bottles fabricated from PET offer impact-resistance, and are less expensive and lighter than glass bottles. In the U.S., the PET bottles have almost completely replaced two liter and larger glass beverage bottles.
PET carbonated-beverage bottles are usually produced by the stretch blow molding process which is designed to impart biaxial orientation in the bottle, wherein such biaxial orientation improves impact resistance, gloss, clarity, and stiffness. The stretch blow molding process is a two step process as described, for example, in U.S. Pat. No. 3,733,309 to Wyeth et al. The first step involves forming a relatively small, thick walled parison having a test-tube shape by, for example, an injection molding process. Subsequently, the resulting parison is reheated to the softening temperature of the parison, i.e., above the glass transition temperature and below the crystal melting temperature of the thermoplastic parison. The reheated parison is placed in a blow mold in which it is stretched to induce vertical orientation and blown to shape. The blowing step induces horizontal orientation to the resulting bottle. The bottle prepared according to the stretch blow molding process does not have the same degree of orientation at every point; however, the areas that are less oriented have a thicker shell than the areas that are more highly oriented. The cylindrical section of the bottle has the thinnest shell thickness. The combination of the shell thickness and orientation provides relatively high overall strength to the resulting bottle.
In addition to the strength requirement, another important property requirement of a carbonated-beverage bottle is the gas barrier property, particularly for carbon dioxide and oxygen, that is needed to preserve the taste of its content during the normal shelf-life. In general, the loss of carbonation of more than about 15% makes a carbonated-beverage unmarketable. Although PET provides sufficient gas barrier properties for large containers, wherein the surface area of the container is small compared to the volume of the content inside, its barrier properties are not adequate for small containers, such as bottles smaller than one liter. This inadequacy is attributable to the high surface to content volume ratio in the small bottles. For example, a 2-liter bottle has about 900 cm.sup.2 of surface area whereas a 1/2-liter bottle has about 350 cm.sup.2 Therefore, a 2-liter bottle has the surface to content volume ratio of about 450 cm.sup.-1 whereas a 1/2-liter bottle has about 700 cm.sup.-1.
There have been many efforts to improve the barrier properties of the PET containers in order to increase the shelf-life of carbonated-beverage products packaged in small containers. Barrier polymer coatings, such as with polyvinylidene chloride, and heat setting to increase the crystallinity of the PET containers are two of such efforts. Another method of improving barrier properties of the PET bottle is described in Japanese Patent 40752 (Aug. 31, 1989). Such patent discloses a post-fabrication extra wrapping layer for PET carbonated-beverage bottles utilizing a heat shrinkable polyester film coated with a polyvinylidene chloride barrier resin. Yet another method of coextruding PET with other barrier resins to form a coextruded bottle has been known in the art. However, such coextruded bottles are not readily recyclable, creating disposal problems.
A novel method of improving the gas barrier properties of PET carbonated-beverage bottles is utilizing a barrier label. As mentioned above, the stretch blow molded bottle does not have a uniform shell thickness. The cylindrical section of the bottle has the thinnest shell thickness and, therefore, this section is most susceptible to the carbon dioxide permeation. In general, the label of the beverage bottle is affixed to and covers most part of this section. U.S. Pat. No. 4,601,926 to Jabarine discloses a method of utilizing a bottle label coated with a heat-activatable, barrier copolyester adhesive resin to impart extra barrier properties to the PET bottle. This process requires that the inside of the label be completely coated with the adhesive and that the entire inner surface of the label be securely bonded to the bottle. Such process is different from the current industry practice where a bottle label is spot or strip tacked with a hot melt adhesive to the bottle and the label overlap is glue sealed.
The most widely used labels for PET carbonated-beverage bottles are two-layer laminations of polypropylene films. The outside layer is a transparent polypropylene layer of about 0.5 mils (13 .mu.m) thickness, which is reverse printed inside, and the inside layer is an opaque polypropylene layer of about 1 mil (25 .mu.m) thickness, which provides the function of accentuating the printed message on the outside layer.
Although such method as the barrier adhesive layer disclosed above provides sufficient barrier properties for larger bottles, it is desirable to have a barrier label system of higher barrier properties that provides high enough barrier properties to be useful for small beverage bottles. It is further desirable to have a barrier label system that does not require changes in the current manufacturing practice of applying the labels to the PET bottles. It is also desirable to have a label system that can be removed in order to facilitate the post-consumer recycling of the bottle.